The present invention relates to an optical reproduction apparatus which carries out the reproduction with respect to an optical recording medium, especially relates to an optical reproduction apparatus that simultaneously carries out (a) an AGC (automatic gain control) in which the gain of an amplifier circuit that amplifies a reproduction signal is controlled and (b) a control of the reproduction power of a light beam.
In an optical disk reproduction apparatus, a reproduction signal detected by an optical pickup is processed such as amplified and wave-form-equalized by a wave form process circuit. The reproduction signal thus processed is sent to a binary circuit or a PRML process circuit so as to be converted into a digital signal. The amplitude of the reproduction signal varies due to the change such as the change in the reflectance of a recording layer. For this problem, in the wave form process circuit, so called an AGC (automatic gain control) is generally carried out so as to stably amplify the reproduction signal and have a predetermined amplitude (see, for example, Japanese unexamined patent publication No. 58-73022 (publication data: May 2, 1983).
FIG. 8 is a diagram showing a structure of a conventional optical reproduction apparatus. The following description deals with the operation of such a conventional apparatus. A reproduction signal read out from an optical disk 30a by an optical head 31 is sent to an AGC amplifier 32 so as to be amplified, and then is sent to an amplitude detection circuit 33. The amplitude of the reproduction signal outputted from the amplitude detection circuit 33 is sent to a gain control circuit 34 which compares the amplitude with a reference amplitude, calculates a gain by which the amplitude of the reproduction signal is made to be equal to the reference amplitude, and returns to the AGC amplifier 32. The reproduction signal thus amplified by the AGC amplifier 32, that has a stable predetermined amplitude, is sent to a digital data reproduction circuit 35. This allows to realize a reproduction of the digital data with high reliance and small reading error. The response speed of the amplifier 32 is usually set to be high so as to swiftly follow the change in the amplitudes of the reproduction signal.
In the mean time, according to a conventional rewritable magneto-optical disk reproduction apparatus of magnetic super-resolution type, two kinds of marks having respective different lengths are reproduced. The reproduction power is controlled so that the ratio of the two reproduction signals becomes close to a predetermined value, thereby ensuring that the reproduction power is always maintained optimal and the reading error is reduced (see, for example, Japanese unexamined patent publication No. 8-63817 (publication date: Mar. 8, 1996). FIG. 9 is a schematic diagram showing a structure of such a conventional rewritable magneto-optical disk reproduction apparatus. FIG. 10 is an explanatory view illustrating a structure of the magneto-optical disk 30. In FIG. 10, a sector 50 is composed of a short mark recording region 51 and a long mark recording region 52 that are provided for controlling the reproduction power, and a data recording region 53. A pattern in which short marks are repeated is recorded in the short mark recording region 51. A pattern in which long marks are repeated is recorded in the long mark recording region 52. Digital data are recorded in the data recording region 53.
In FIG. 9, the light projected from a semiconductor laser 36 is converged onto the sector 50. When the light is converged onto the short mark recording region 51, the light reflected from the pattern in which the short marks are repeated is directed to a photo diode 37 by which the reflected light is converted to a reproduction signal. In a similar manner, the reproduction signal of the pattern in which the long marks are repeated is reproduced from the long mark recording region 52. The reproduction signals are sent to an amplitude ratio detection circuit 38. The amplitude ratio detected by the amplitude ratio detection circuit 38 and a target amplitude ratio are compared with each other by a differential amplifier 39. And, a feedback control is made, i.e., a laser power control circuit 40 controls a driving electric current of the semiconductor laser 36 so that the difference between the above ratios becomes small. Thus, the driving electric current of the laser light is controlled so that the optimal reproduction power is provided. Thereafter, the projected light is converged onto the data recording region 53. The reproduction signal read out from the data recording region 53 is sent to a digital data reproduction circuit 41 which reproduces the digital data having a low error rate. Thereafter, when converging the projected light onto the next sector, the processing is repeated in a similar manner so that an optimal reproduction power is newly set. Thus, the recording regions for the marks of a reproduction power control use are provided in a dispersion manner, and the amount of the reproduction signal is detected for each sector so as to control the reproduction power. This allows to control the reproduction power in a short time interval and to follow the change in the optimal reproduction power in a short time.
By the way, in the reproduction power control, the error is generated in the process of the detection of the amplitude ratio. More specifically, even when reproducing the mark pattern of the reproduction power control use under the same reproduction power and the same conditions, generated is the unevenness of the amplitude ratio that is reproduced by the noise contained in the reproduction signal. This causes the error in the differential signal of the detected amplitude ratio and the target amplitude ratio. The error is generated in the reproduction power to be controlled, accordingly. Even when the reproduction is made under the same conditions, since the reproduction power to be controlled has the unevenness due to the error. The amplitudes of the reproduction signal have also the unevenness, accordingly. For example, in the case where the reproduction power is controlled for each sector like the reproduction apparatus shown in FIG. 9, the reproduction conditions should be almost the same among the adjoining sectors. However, in actual, the amplitudes of the reproduction signals obtained for each sector have the unevenness due to the above-mentioned reason. Note that the amount of the unevenness is not so large. The error rate in the digital data reproduction is little affected.
However, in the case where the reproduction power control and the AGC are simultaneously carried out, when the response speed of the AGC is so fast as to carry out the AGC for each sector, as has been discussed, the gain of the AGC is found in accordance with the amplitude that varies depending on the unevenness of the reproduction power. This causes the problem that the unevenness of the amplitudes becomes larger so as to adversely affect the digital data reproduction.
FIG. 11 shows the results of the change in the amplitudes of the long mark that has actually been measured when controlling the reproduction power for a single sector under a constant condition. The horizontal axis indicates how many times the controlling has been carried out (how long the time has passed). The vertical axis indicates the amplitudes of the reproduction signal of the long mark. FIG. 11(a) shows the change in the amplitudes when only the reproduction power control is carried out. FIG. 11(b) shows the change in the amplitudes when the reproduction power control and the AGC for each sector are simultaneously carried out. As is clear from FIGS. 11(a) and 11(b), when the reproduction power control and the AGC for each sector are simultaneously carried out, the unevenness of the amplitudes became large. When comparing the both cases in accordance with the standard deviation ("sgr"), the standard deviations are 13 percent for the result shown in FIG. 11(a) while 22 percent (becoming worse) for the result shown in FIG. 11(b).
Note that since it is not possible without the AGC to follow the amplitude when the reflectance of the recording layer changes, for example, the problem still remains that the error rate deteriorates for the digital data reproduction.
It is an object of the present invention to provide an optical reproduction apparatus in which the reproduction power control of a light beam and the automatic gain control are simultaneously carried out.
In order to achieve the foregoing object, an optical reproduction apparatus in accordance with the present invention is characterized by being provided with (a) gain control means for controlling a gain of an amplifier circuit that amplifies a reproduction signal from an optical recording medium in accordance with amplitudes of the reproduction signal, and (b) power control means for controlling reproduction power of the light beam in accordance with the amplitude, wherein the gain control means has a slower response speed than that of the power control means at least during a period when the power control means controls the reproduction power.
With the arrangement, the AGC gain does not follow the change in the amplitudes due to the control error of the reproduction power, but can only follow the change in the amplitudes whose time constant is great due to the difference of recording conditions or other reasons.
By averaging the amplitudes from which the gain of the AGC is found and using an averaged amplitude, the gain of the AGC does not locally follow the change in the amplitudes, but can only follow the change in the amplitudes whose time constant is great due to the difference of recording conditions or other reasons.
Further, since the gain having little error is found in accordance with an averaged amplitude that is obtained by averaging only the amplitudes detected normally, it is possible to realize an AGC with high accuracy.
In addition, when (a) monitoring how the reproduction power controls, and (b) switching to the slower response speed of the AGC during a period when the reproduction power is controlled while (c) switching to the higher response speed of the AGC during a period when the reproduction power is not controlled, it is possible to realize an AGC that are suitable for respective cases with high accuracy.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitative of the present invention.